In a waveguide system having a source and a load, it is desirable to transfer all of the energy generated by the source to the load. If a mismatch exists in the waveguide system, a standing wave is established in the waveguide and some of the energy is reflected back to the source. The standing wave can be canceled at any particular frequency by introducing an additional mismatch elsewhere in the waveguide system whose reflection coefficient is in anti-phase with that of the original mismatch.
Typically, a stub tuner or a matching section is used to introduce the additional mismatch into the system. Stub tuners are capable of providing an impedance match between a source and a load over a broadband of frequencies. Stub tuners typically consist of lengths of waveguide with either a single dielectric or metallic stub which is variable in position along the waveguide or multiple stubs at fixed positions in the waveguide whose penetrations into the waveguide can be individually varied. The stubs typically provide a variable capacitive susceptance.
Stub tuners produce a variable mismatch with a variable phase. For a single stub tuner, a variable mismatch is achieved by varying the insertion of the stub into the waveguide and a variable phase is produced by varying the position of the stub along the axis of the waveguide. For a multi-stub tuner, a variable phase is achieved by inserting two or more stubs with a particular spacing into the waveguide. Theoretically any mismatch could be canceled using three stubs, although four or five stubs are occasionally utilized to achieve greater versatility.
In microwave systems having a source and a load, it is desirable to have a multi-stub tuner that automatically adjusts the stubs to positions which establish an additional mismatch in the system which results in the source impedance matching the load impedance. To achieve an impedance match between the source and the load, the additional mismatch must have a reflection coefficient that is in anti-phase with that of the original mismatch. Such a multi-stub tuner is particularly desirable for microwave plasma systems because the state of the plasma and thus the impedance of the plasma may change rapidly. If automatic impedance adjusting is performed fast enough, a stable plasma can be achieved for various conditions.
U.S. Pat. No. 5,079,507 discloses an automatic impedance adjusting apparatus and methods for microwave systems. This patent discloses an apparatus and methods which initially measure either an impedance or a reflection coefficient associated with a microwave load at a predetermined reference point on a microwave waveguide connecting the source to the load. Then a variable impedance apparatus, such as a three-stub tuner, positioned on the load side of the reference point, is adjusted so that the impedance associated with the microwave load is a predetermined value of impedance. This predetermined value of impedance is calculated to match the impedance of the source to the impedance of the load or to achieve a predetermined value of mismatch. This method is typically repeated until the source impedance actually matches the impedance of the load or the predetermined value of mismatch is achieved.
The apparatus and method described in the patent may rapidly match the source impedance to the load impedance when the load impedance is fixed. Unfortunately, many loads, such as plasmas, have an impedance that is a nonlinear function of the power delivered to the load. When such apparatus and methods are utilized with such nonlinear loads, the variable impedance apparatus may iterate without converging (this phenomena is known as hunting in servo-mechanical systems) or may slowly but erratically converge to an impedance which matches the impedance of the source to the impedance of the load. The nonconvergence or slow, unstable or erratic convergence, may result in an unstable plasma and may even cause the plasma condition to terminate. In a semiconductor plasma processing apparatus, this is particularly problematic since having a stable plasma is critical to achieving uniform etching and thin film deposition rates.
It is therefore a principal object of this invention to provide an impedance matching apparatus and method which rapidly adjusts the impedance of a variable impedance apparatus to match the impedance of a source to the impedance of a load, where the load impedance is a nonlinear function of the power delivered to the load. It is another object of this invention to provide an impedance matching apparatus and method which maintain a stable plasma. It is another object to accurately model a multi-stub tuner at high voltage standing wave ratios using scattering parameters.
It is another object to provide a photosensitive detector and a spectrometer or a filter for indicating that a plasma or a plasma chemistry exists. Other objects are to provide a look-up table for determining the appropriate stub positions for initially establishing the plasma or the plasma chemistry and a second look-up table for matching the source impedance to the load impedance when the photosensitive detector indicates that the plasma or the plasma chemistry exists.